Comments: (37 pages, A4, 10pt, 1 figure) This is a complete version of a preliminary report, which appeared in the Proceedings of the 7th International Conference on Combinatorial Optimization and Applications (COCOA 2013), Chengdu, China, December 12--14, 2013, Lecture Notes in Computer Science, Springer-Verlag, vol.8287, pp.318--329, 2013